macrumors 65816

This is just an option which might help, go to preferences to screens, and drag from the little blue screen representing your macbook's screen, the white bar to the bleu screen representing your ACD. Now the ACD is your main screen. I think iTunes should show the movie fullscreen on your acd now.

macrumors 6502a

I haven't upgraded to iTUnes 9 yet so this is just from my experience with projectors and external monitors in general.

Sometimes programs will only play certain videos on the "Main" screen, ie: when using PowerPoint with a video imbedded, the video will only pay on the laptop screen and a black box appears on the projector screen. Swapping the screens so that the projector is the #1 or main screen fixes that. (It's a PITA in the middle of a lecture when the person speaking brings their own Windows laptop and you've got to fix it...)

Like arjen92 said, drag the white bar to the ACD screen is the Monitor System Prefs. That should fix it and when you disconnect the laptop should be back to normal.

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